A Neutral Beryllium(I) Radical

Abstract The reduction of a cyclic alkyl(amino)carbene (CAAC)‐stabilized organoberyllium chloride yields the first neutral beryllium radical, which was characterized by EPR, IR, and UV/Vis spectroscopy, X‐ray crystallography, and DFT calculations.

Abstract: The reduction of a cyclic alkyl(amino)carbene (CAAC)-stabilized organoberyllium chloride yields the first neutral beryllium radical, which was characterized by EPR, IR, and UV/Vis spectroscopy, X-ray crystallography, and DFT calculations.
While group 2 chemistry is mainly dictated by the naturally occurring + 2 oxidation state of its elements, the last two decades have seen the emergence of a growing number of low-oxidation-state alkaline earth metal compounds. Since the landmark synthesis of the first dinuclear Mg I complexes, including I (Figure 1), [1] these compounds have been successfully applied as highly selective reducing agents for the activation of small molecules [2] and the synthesis of new homo-and heteronuclear metal À metal bonds, [3] culminating most recently in the isolation of the first molecular Mg 0 species, complex II. [4] Low-valent beryllium complexes long remained confined to the computational realm due to their very high toxicity. Recent years, however, have seen a renewed interest in beryllium coordination chemistry in the areas of organometallic, pure inorganic, and bioinorganic chemistry. [5] Although the viability of Be I À Be I -bonded species has been predicted, [6] and the Be 2 dimer has been observed spectroscopically, [7] the low BeÀBe bond enthalpy makes mononuclear Be 0 compounds more accessible targets. [8] In 2016 our group reported the first Be 0 compound, complex III, which owes its stability to strong three-center-two-electron p backbonding from the Be 0 atom in its 2s 0 2p 2 electronic configuration to the neutral cyclic alkyl(amino)carbene (CAAC) ligands, [9] and this has since been used as a reducing agent to synthesize the first carbene bismuthinidene complex. [10] Although beryllium radicals have been postulated as intermediates in reduction reactions resulting in ligand activation, [11] the first isolable Be I radical cation, IV, was only reported in 2020 from the oneelectron oxidation of an analogue of III with 2,2,6,6-tetramethylpiperidin-1-oxyl. [12] Calculations showed that the bonding in IV is similar to that in III, with two neutral CAAC ligands stabilizing a Be I cation in its excited 2s 0 2p 1 electronic configuration through donor-acceptor interactions, and that the spin density is delocalized over the entire N-C-Be-C-N framework, with 38 % located at the beryllium center. Furthermore, Paparo and Jones succeeded in isolating the first neutral Be I complexes, such as V, which present covalent Be I ÀAl I bonding. [13] We now report the synthesis and computational analysis of the first structurally characterized neutral beryllium radical, stabilized by both a neutral and a C1-protonated CAAC ligand.
Whereas the reduction of (CAAC)(CAACH)BeBr with a wide range of reducing agents in various solvents at best resulted in partial reduction and the formation of [CAACH]Br as the sole isolable product, the room-temperature reduction of (CAAC)(CAACH)BeCl with lithium sand in diethyl ether over a period of 10 minutes resulted in the formation of the radical species [(CAAC)(CAACH)Be]C, which was isolated as a brown-orange crystalline solid in 70 % yield (Scheme 1 b). While the radical proved stable in the solid state at À30 8C under an argon atmosphere for several weeks, it decomposed within minutes in polar solvents, such as THF and 1,2-difluorobenzene, and within two days in diethyl ether at À30 8C. In aromatic hydrocarbon solvents, such as benzene and toluene, the compound was less soluble but remained stable at room temperature, provided silanized glassware or polyethylene vials were used to avoid its reaction with glassware surface OH groups. Under these conditions [(CAAC)(CAACH)Be]C could be heated up to 60 8C before significant decomposition set in.
As expected, [(CAAC)(CAACH)Be]C was NMR-silent but displayed a complex EPR signal centered at g iso = 2.003. Simulation provided a hyperfine coupling constant to 9 Be of 11.6 MHz (4.1 G, Figure 2 a), significantly larger than for IV (0.32 G). [12] Calculations at the UBP86-D3(BJ)/def2SVP level of theory, performed using Gaussian 16, [17] show that the SOMO is mainly delocalized over the Be À C CAAC p bond, with some p-antibonding character on the C À N bond of the CAAC ligand (Figure 2 b), as is the case for most CAAC-stabilized main group radicals. [18] The calculated spin density at beryllium (23 %) is significantly lower than that calculated for the radical cation IV (38 %). [12,19] The solid-state IR spectrum of [(CAAC)(CAACH)Be]C shows a characteristic band at 2693 cm À1 which calculations attribute to the CÀH stretching frequency of the protonated beryllium-bound carbon atom (ñ calcd = 2725 cm À1 ). The UV/ Vis spectrum of [(CAAC)(CAACH)Be]C, which had to be recorded in Et 2 O in a silanized cuvette to avoid decomposition, shows a broad absorption centered at l max = 350 nm, spanning over 100 nm at mid-height and extending into the 400-500 nm range, thus accounting for the brown-orange coloration of the radical. [20] Accordingly, TD-DFT calculations indicate the presence of charge-transfer transitions from the SOMO to low-lying LUMOs in this wavelength window, the one with the largest oscillator strength appearing at 330 nm (UCAM-B3LYP/6-31 ++ G**, see Supporting Information).
The energy decomposition analysis in combination with the natural orbitals for chemical valence method (EDA-NOCV), as implemented in ADF 2019, [22] was applied to [(CAAC')(CAAC'H)Be]C (truncated model with Me and iPr groups replaced by hydrogen atoms) in order to investigate its bonding situation. The results were obtained at the BP86-D3(BJ)/TZV2P level of theory. The quantitative results for three distinct decomposition schemes are shown in Table S1 in the Supporting Information. These were based on [(CAAC'H)Be]C and CAAC' as interacting fragments and varied depending on the electronic configuration and multiplicity of the fragments. The interaction between [(CAAC'H)-Be]C in its first excited doublet configuration, where the radical occupies a p ? orbital of Be, and a ground-state singlet CAAC' resulted in the lowest orbital interaction term DE orb . As this is a useful criterion for discerning the best bonding description in terms of interacting fragments, [15,23] we conclude that donor-acceptor interactions are at play in the stabilization of the [(CAAC')(CAAC'H)Be]C neutral radical.
As shown in Table S1 in the Supporting Information, essentially half of the attraction (50.3 %) between the [(CAAC'H)Be]C and CAAC' fragments is due to the covalent contribution DE orb . The dispersion contribution (DE disp ) accounts for merely 2.6 % and the electrostatic attraction DE elstat is responsible for the remaining 47.1 %. These results are comparable to those observed for the paramagnetic beryllium radical cation IV. [12] The breakdown of DE orb into pairwise orbital interactions (Table SX and Figure 3) shows that the strongest contribution comes from the (CAAC'H)Be!CAAC' p backdonation from the Be radical into the vacant p orbital of the CAAC' ligand (DE orb(2a) = À56.5 kcal mol À1 , see Figure 3 for the corresponding deformation density). This contribution is slightly stronger than that obtained for the radical cation IV [12] and significantly weaker than that of the neutral Be 0 species III, [9] the latter on account of the half-empty Be p ? orbital of [(CAAC')-(CAAC'H)Be]C, which is a weaker donor in comparison to the doubly occupied Be p ? orbital of III. In contrast, the (CAAC'H)Be ! CAAC' s donation (see Figure 3 for the corresponding deformation density) is DE orb(1ab) = À43.4 kcal mol À1 , weaker than the CAAC!Be ! CAAC s donation contributions in III and IV. This is explained by the fact that in this case only one CAAC ligand contributes to the s donation, while in the previous cases both ligands donate to the central Be atom.
In order to assess the possibility of fluxional hydrogen shifting from CAACH to CAAC in [(CAAC)(CAACH)Be]C via an intermediate tricoordinate tautomer [(CAAC) 2 BeH]C, we also examined the latter computationally (Figure 4). At the UBP86-D3(BJ)/def2SVP level of theory [(CAAC) 2 BeH]C lies 10.5 kcal mol À1 higher in energy than [(CAAC)-(CAACH)Be]C. Its SOMO is p-delocalized along the two CAAC ligands and the central Be atom and features two nodal planes at the C À N bonds. In contrast, the LUMO of [(CAAC) 2 BeH]C, which is mostly located at the endocyclic CÀ N bonds, is stabilized by 0.56 eV compared to that of [(CAAC)(CAACH)Be]C. This leads to a decrease in the SOMO-LUMO (SL) gap of [(CAAC) 2 BeH]C to merely 0.75 eV, less than half the SL gap of [(CAAC)(CAACH)Be]C (1.61 eV). These results show that a doubly CAAC-stabilized BeH radical is not energetically accessible. Since the nature of the Lewis base (L) strongly influences the electronic and structural features of main group compounds, [24] a theoretical investigation of various [L 2 BeH]C radicals, aiming at the  identification of potential synthetic targets, is currently under investigation in our group.
To summarize, we have synthesized and structurally characterized a stable neutral Be I radical, the first example of an isolable neutral s-block radical with significant spin density located at the metal center. We are currently investigating the reactivity of this species and will report our findings in due course.